Powerplants for large aircraft generally include a turbofan, gas turbine engine and a nacelle for the engine. The nacelle extends circumferentially about the engine, sheltering the engine and providing aerodynamic surfaces which reduce aerodynamic drag on the powerplant as it generates thrust.
The turbofan engine includes a compression section, a combustion section, and a turbine section. A primary flowpath for working medium gases extends axially through these sections of the engine. An engine casing extends axially through the engine and circumferentially about the flowpath to bound the primary working medium flowpath.
The working medium gases of the primary flowpath are drawn into the compression section where they pass through several stages of compression, causing the temperature and pressure of the gases to rise. The gases are mixed with fuel in the combustion section and burned to form hot, pressurized gases. These gases are a source of energy to the engine and are expanded to the turbine section to produce work.
A secondary flowpath for working medium gases is disposed outwardly of the primary flowpath. The secondary flowpath is annular in shape. The engine has a plurality of fan blades which extend radially outwardly across the primary flowpath and secondary flowpath. These fan blades pressurize working medium gases entering both flowpaths of the engine.
The nacelle includes a fan nacelle and a core nacelle. The fan nacelle extends outwardly of the fan blades. The core nacelle is spaced inwardly from the fan nacelle, leaving the secondary flowpath for working medium gases therebetween. The core nacelle is spaced radially outwardly from the engine, leaving a core nacelle compartment therebetween which extends about the gas turbine engine.
The core nacelle compartment provides an enclosed shelter for components (commonly called engine accessories) which are mounted on the exterior of the engine. These accessories might include fuel lines for flowing fuel to the combustion section; an electrical generator for supplying the engine and aircraft with electrical power; a pneumatic duct for ducting a portion of the compressed gases of the engine through the nacelle and through struts to the aircraft; a second pneumatic duct for ducting compressed gases to the engine during start up from an auxiliary power unit (commonly referred to as the "APU") located in the aircraft. Other components are a hydraulic pump for providing pressurized hydraulic fluid to hydraulic vane actuators, and heat exchangers for removing heat from high-temperature fluids such as the engines lubricating fluid, fuel, and other fluids.
Cooling is provided to all of these components to keep the temperature of the components within acceptable limits. As will be realized, a primary source of heat is the engine, which employs a combustion process having temperatures which exceed twenty-five thousand (2,500.degree. F.) degrees fahrenheit. As a result, while some portions of the accessories on the engine may have a temperature as low as two hundred (200.degree. F.) degrees fahrenheit, other exterior portions of the engine and accessories have temperatures in excess of one thousand (1,000.degree. F.) degrees fahrenheit. Accordingly, the nacelle compartment is ventilated during engine operation by cooling passages which flow a portion of the cool pressurized air from the secondary flowpath to the interior of the nacelle for cooling.
One example of a cooling passage is an axially extending spray bar at the top of the nacelle or at the sides of the nacelle. The spray bar flows cooling air into the upper region or side regions of the nacelle to ventilate the core nacelle compartment during operation of the engine. Another example of a cooling and ventilating system used in gas turbine engine nacelles is shown in U.S. Pat. No. 4,019,320 entitled External Gas Turbine Cooling for Clearance Control and U.S. Pat. No. 4,069,662 entitled Clearance Control for Gas Turbine Engine both issued to Redinger et al. and assigned to the assignee of this invention. In these constructions, cool air is led from the fan discharge duct and is directed externally of the engine case into a region adjacent to seals in the turbine section of the gas turbine engine. Spray bars impinge the cooling air on the engine case to control the diameter of the case and internal operating clearances on the engine which are associated with the position of the engine case. After the cool air impinges on the engine case, the air is flowed through the engine compartment to ventilate the compartment.
Another example of a cooling passage is shown in U.S. Pat. No. 4,351,150 issued to Schulze entitled Auxiliary Air System for Gas Turbine Engine. In Schulze, an air ducting pipe is used to duct cooling air to a component which generates heat such as an electronic engine control to cool the component. After cooling the control, the air is discharged into a compartment of the nacelle to provide ventilation to the compartment.
Another example of a cooling system for the nacelle is shown in U.S. Pat. No. 5,127,222 issued to Ream et al. entitled Buffer Region for the Nacelle of a Gas Turbine Engine. In Ream et al., cooling air is flowed to a buffer region in the nacelle after engine shutdown. The cooling air surrounds the component after engine shutdown to avoid transfer of the heat from hot gases on the interior of the compartment to the component to keep the temperature of the hydraulic fluid within acceptable limits.
There is a second problem caused by the generation of heat by the engine during operation. Failure of a component requires that the component be replaced on the wing and in a short period of time. This is particularly a problem if the component is a dispatch critical item. After a dispatch critical item fails during the flight of the aircraft, the dispatch critical item must be repaired or replaced before the aircraft is permitted to continue with its regularly scheduled service. This is true even though a back-up system even though the back-up sensing system is functioning perfectly, the aircraft cannot take off until the primary sensing system is repaired (for example, main oil pressure sensing system is functioning perfectly).
Thus, the dispatch critical item must be repaired or replaced prior to departure of the aircraft which normally takes place in one to two hours. Items such as electrical connectors, pressure diaphragm sensors, components such as hydraulic actuators, electrical generators, heat exchangers are designed to be replaceable within thirty (30) minutes to ensure the aircraft meets its scheduled departure time. For example, parts are not layered to promote access to the parts and to provide spacing between the parts so that easy access is provided. Back-up wires have been eliminated from conduits carrying air to speed replacement of these components by engine maintenance personnel. Hinge clamps are employed in many locations and other designs of attaching devices to the engine are to decrease the time to remove and replace these components.
Engine maintenance personnel are provided with protective clothing, including gloves to shield them from hot components during removal and replacement of a malfunctioning component. As a result of these protective devices, the maintenance personnel are able to work on even the hottest components as soon as the components cool down to a temperature at which the protective clothing is effective.
The above art, notwithstanding, scientists and engineers working under the direction of applicant's assignee have sought to reduce the amount of time required to remove and replace components on the exterior of the engine to ensure that turn time for the repair of such components is less than one to two hours to ensure that the aircraft can meet is scheduled departure time.